If your development times are short you may need to pre-wet to get more even results (or so I've been told) - with any developer, not just HC-110. If the mottling is very white/grey, it is likely that your film was touching itself and it didn't get fixed (or developed) properly. If the mottling is in area like the sky, where you have a large even area of the same tonal value, then it is possible that there was too much fluid in the can and not enough agitation happened.

There are any number of reasons, but if you could attach a thumbnail image it would help narrow it down.

It was 35mm film. I presoaked for two minutes with agitation to eliminate the purple dye. Agitated 30 - 60 seconds (don't remember), then for 10 seconds every minute (gently), then used a water stop bath. Fixed for 5 minutes in Ilford non-hardening fixer. Looks more like grain. Never mind the composition/exposure...it was just an experiment.